Permeation And Diffusion Behaviors Of Hydrogen And Its Isotope In Molten-salt

As one of the six candidates for the Generation IV nuclear reactors, Molten SaltReactor (MSR) is the only one using liquid fuel, and it can be used to address some ofchallenges facing the thorium resources utilization in nuclear power, which leads theMSR to be the ideal reactor for thorium (Th)-uranium (U) fuel cycle, in whichthorium fuel can breed fissile233U. The development and construction project ofThorium based Molten Salt Reactor (TMSR) was put forword, with the appropriatechoise for the demand of the nuclear energy development and the effective utilizationof thorium resources.In MSR, the fuel salt contains a large quantity of lithium, which can generatetritium by absorption of neutrons. At high temperatures, tritium has high permeability,and can diffuse through almost all of metallic meterials. In MSR, tritium tends todiffuse through the metal walls of the various systems, and transfer to theenvironment and the steam system, which can cause harm for human body. Thereby,tritium must be controlled in MSR. The behaviors of tritium in molten salts are ofimportant significance for tritium control in MSR. As the isotopes of tritium,hydrogen and deterium can be used as the substitute of tritium to study its behaviorsand characteristics in molten salts.The PhD thesis project is supported by "Future Advanced Nuclear FissionEnergy-Thorium based Molten Salt Reactor (TMSR)" project of Shanghai Institute ofApplied Physics, Chinese Academy of Science. The main works of this paper are asfollows:1. Designing and buliding a testing system for determining the permeationand diffusion characteristics of hydrogen isotopes in molten salts;2. Using the testsystem, studying the behaviors of hydrogen isotopes in molten salts, hence providingsome references for the research of tritium behaviors, and its properties in moltensalts.1. The testing system for permeation and diffusion characteristics of hydrogenisotopes in molten salts.The testing system can be divided into functional parts of the gas supply system,permeation pot and heating system, diagnostics system, and auxiliary components. The permeation pot is a main part of the testing system. When performing anexperiment, the molten salt was initially set in the permeation pot, and H2(or D2) wasloaded in one side of the molten salt. At high temperatures, H2(or D2) could penetratethe molten salt and arrived at the other side of the molten salt. By the processes of thetransient-state permeation and the steady-state permeation of H2(or D2) in molten salt,one could obtain the relevant parameteres for the solution and diffusion behaviors ofH2(or D2) in molten salt.Experiments of H2permeation in molten Flinak (LiF-NaF-KF)(10mm inthickness) were conducted at500°C with various H2partial pressures in H2load side,and at500-700°C with H2partial pressures of1.0atm in H2load side. It indicated thata part of H2could permeate through the side wall of the permeation pot and pass intothe outside chamber, which could permeate again through the side wall of thepermeation pot and pass into the other side of the molten salt. This by-passpermeation phenomenon can cause larger experiment result errors, and effect theprocess of permeation experiment, and increase the difficulty of the subsequentcalculation. By sweeping or vacuum for the outside chamber, the impact of H2by-pass permeation on the experiment can be reduced effectivity.The feasibility studies of the testing system were conducted through the methodof vacuum for the outside chamber. Performance evaluation experimentes for thenickel over300-700°C show that the properities of nickel effect less on the hydrogenisotopes permeation in molten salts. By the test experiments of H2in Molten Flinak(30mm in thickness) with500°C,600°C and700°C, it showes that the results, derivedfrom the experiment of hydrogen isotopes permeation in molten salt, can becalculated, and match with the hydrogen isotope permeation models.2. The permeation experimentes of hydrogen isotope in molten salt.The thesis conducted the permeation experimentes of H2in molten Flinak(30mm in thickness) under500°C,600°C and700°C, and various hydrogen partialpressures in H2loading side. It indicated that by loading H2in lower chamber, H2inmolten Flinak permeated mainly in hydrogen ions form, which can change tohydrogen molecular form when loading H2in upper chamber.The thesis conducted the permeation experimentes of H2in molten Flinak (30mmin thickness) under500-700°C, and1.0atm of H2loading pressure. Hydrogendiffusivities and solubilities in molten Flinak were obtained under the conditions ofloading H2in lower chamber, and loading H2in upper chamber. By comparing each other, larger diffusivity and lower diffusion activiation energy occurred in thehydrogen molecular form permeation process, where the solubilities were close tothose in the hydrogen ions form permeation process.The thesis conducted the permeation experimentes of D2in molten Flinak(30mm in thickness) under500-700°C, and1.0atm of D2loading pressure. Deteriumdiffusivities and solubilities in molten Flinak were obtained under loading D2in lowerchamber. Comparied with H2, no obvious difference can be found in diffusivity andsolubility.As a result, when conducting the permeation experiments of hydrogen isotopesin molten salts with the testing system, the hydrogen (deterium) loading methodscould effect the results of the permeation experiments, and cause different diffusivitiesof hydrogen isotopes in molten salts. When conducting the permeation experiments ofhydrogen and deterium in molten salts, it showed that there were not significantdifferences of the diffusion and permeation behaviors between hydrogen and deteriumin molten salts. It indicated that the characteristics of hydrogen isotopes had littleinfluences in its diffusion and permeation behaviors in molten salts, which providedexperimental basis for using hydrogen and deterium as the substitute of tritium tostudy their behaviors and characteristics in molten salts.